As a method or means for processing a sheet metal in a 3-dimensional shape, there has been known a forming method in which a metal sheet is fixed on an X-Y table and pressed down by a tool disposed in an upper position and movable in a Z-axis direction. In this case, the tool is moved to sequentially generate plastic deformation in the metal sheet. In an application of this method, a forming model may be adopted. FIG. 1 shows an outline of this forming method.
However, the conventional technique has the following shortcomings because the entire shape to be formed is sequentially formed by drawing a contour line with a bar-like tool.    1) The forming requires much time until the end of the work. Therefore, it is difficult to provide a sufficient amount of commercialized products, such as 500 pieces per a month.    2) A sheet thickness is significantly reduced.
Assuming that the thickness of a blank sheet is t0, the sheet thickness after the forming is t, the forming angle is θ, and the thickness reduction ratio is δ, a sheet thickness after the forming can be obtained by t=t0·sin θ. Conventionally, if the forming angle is 20° and the sheet thickness is 0.8 mm, the thickness reduction ratio becomes 34.2%, so that a portion having the reduced thickness may be susceptible to breakdown. For example, if the sheet metal is used in a car component, the thickness reduction ratio should be within 30%. However, this requirement cannot be observed by using the conventional forming technique.    3) It is difficult or impossible to form a vertical wall.
If there is an abrupt angle change in the formed product, it is difficult to perform forming to model this shape. The forming angle θ is limited to 15° in aluminum, 20° in SPC, and 25° in SUS. Therefore, in addition to 2), there is a limitation to the shape that can be formed.    4) The surface finishing is not good.
Since the forming is performed by moving the bar-like tool along a contour line, a moire shape tool trace is inevitably generated. If a pitch is made minute to reduce this tool trace, the forming process takes longer time.    5) Accuracy is insufficient.
The conventional art is a method of using only “extension” of a material. Therefore, the finished product may be poor in sheet thickness reduction or numerical accuracy in comparison with a press-formed product.
In order to solve such problems, the inventors have proposed, in Japanese Patent Unexamined Application Publication No. 2003-53436, a method in which rough forming is performed by pushing a forming punch having a desired shape in the thickness direction of a blank workpiece with its edge being clamped, and then shape-forming is performed with a bar-like tool from the opposite direction while the blank workpiece are held between the forming punch and the bar-like tool, with the forming punch being pushed in.
According to this method, the aforementioned problem has been considerably alleviated. However, even in this method, as shown in FIG. 2, the entire workpiece is roughly formed in one time by pushing the forming punch having a shape to be formed in the sheet thickness direction with the entire stroke, and then, detailed forming is performed in this state. Therefore, if there is a recess A in the shape to be formed as shown in FIG. 2, body wrinkles BS are inevitably generated due to a redundant material as shown in FIG. 3. As a result, product accuracy may be degraded.